1. Field of the Invention
This invention relates generally to a method of manufacturing a disintegrable core for castings. More particularly, the present invention relates to a method of manufacturing a disintegrable core for castings, wherein the core so produced is especially characterized by excellent strength and disintegratability.
2. Description of the Prior Art
In the pressure die casting method, a molten metal is placed into a mold under pressure. According to this method, a core which is used in the process must have sufficient strength to tolerate unequal pressure which is generated when the mold is being filled with the molten metal.
A conventional core for the pressure die casting method is made of melted moldings of a mixture of sodium silicate (Na.sub.2 O.SiO.sub.2) and sodium disilicate (Na.sub.2 O.2SiO.sub.2). While such a core has a sufficient strength, when used in the production of aluminum castings a core containing this composition corrodes the aluminum casting products due to the strong alkalinity of the water solution thereof. Therefore, a core containing this composition cannot be used in the casting of aluminum or zinc products.
Another conventional core of this type is disclosed in Japanese Patent Publication No. 56-22420. This core is made by mixing an organic binding material into a fireproof granular substance, molding the mixture of the organic binding material and the fireproof granular substance to a prescribed shape, impregnating the moldings with water glass, prepared with sodium oxide (Na.sub.2 O) at a concentration of 1.2 to 2.2%, and then heating the impregnated moldings. The casting products prepared with this core can be prepared with good quality, even if aluminum or zinc products are cast, as the casting products of aluminum or zinc are not corroded by this core. Unfortunately, when casting an aluminum piston with such a core, good results cannot be obtained due to the non-disintegratability of this core.
An additional conventional method of manufacturing a core is disclosed in Japanese Patent publication 56-22420. While the core in this publication is described as being disintegratable, a hard condition for heating is mandatory. Temperatures typically in excess of 1300.degree. C. for about 24 hours duration are required.
Therefore, a need continues to exist for a core for castings which does not corrode cast products of aluminum or zinc, which also has sufficient strength and which is also disintegrable. Also, it would be extremely desirable to be able to produce such a core with lower temperatures for shorter durations to reduce the expenditure of energy.